1. Field of the Invention
The present invention relates to a process for the preparation of polymers of alpha-olefins, especially homopolymers of ethylene and copolymers of ethylene and higher alpha-olefins having improved colour. In particular the invention relates to a solution process for the preparation of polymers of alpha-olefins in which alpha-olefin monomer is polymerized in the presence of a coordination catalyst capable of being used at relatively high polymerization temperatures, especially temperatures above 150.degree. C. The activator of the coordination catalyst is an aluminoxane.
2. Description of the Prior Art
Polymers of ethylene, for example, homopolymers of ethylene and copolymers of ethylene and higher alpha-olefins, are used in large volumes for a wide variety of end-uses, for example, in the form of film, fibres, moulded or thermoformed articles, pipe, coatings and the like.
There are two types of processes for the manufacture of polyethylene that involve the polymerization of monomers in the presence of a coordination catalyst viz. those which operate at temperatures below the melting or solubilization temperature of the polymer and those which operate at temperatures above the melting or solubilization temperature of the polymer. The latter are referred to as "solution" processes, an example of which is described in Canadian Patent No. 660 869 of A. W. Anderson, E. L. Fallwell and J. M. Bruce, which issued Apr. 9, 1963. In a solution process the process is operated so that both the monomer and polymer are soluble in the reaction medium. Under such conditions accurate control over the degree of polymerization, and hence the molecular weight of the polymer obtained, is achieved by control of the reaction temperature. It is believed that the termination reaction controlling the molecular weight of the polymer is highly dependent on temperature. The molecular use of relatively small amounts of hydrogen, as is described in Canadian Patent No. 703 704 of C. T. Elston, which issued Feb. 9, 1965.
There are a number of advantages to a solution polymerization process, for example, the ability to control the molecular weight of the polymer obtained, the ability to operate the process as a continuous process and to recover the polymer by precipitation without the need for washing, the efficient use of catalyst, the properties of the polymer obtained and the possibility of efficient use of energy.
A disadvantage of a solution process is that part of the catalyst remains in the polymer of ethylene. Such catalyst, which may be referred to herein as "catalyst residue", may contribute to the colour of the polymer obtained and to degradation of the polymer during subsequent processing of the polymer e.g., in extrusion, injection moulding and the like, and/or on exposure of fabricated articles to ultra violet light. The amount of catalyst residue is related, at least in part, to the overall activity of the catalyst employed in the polymerization step of the process as the higher the overall activity of the catalyst the less catalyst that is, in general, required to effect polymerization at an acceptable rate. Catalysts of relatively high overall activity are therefore preferred in solution polymerization processes.
Two important factors in determining the overall activity of a catalyst are the instantaneous activity of the catalyst and the stability of the catalyst under the operating conditions, especially at the operating temperature. Many catalysts that are stated to be very active in low temperature polymerization processes also exhibit high instantaneous activity at the higher temperatures used in solution processes. However such catalysts tend to decompose within a very short time in a solution process and thus the overall activity is disappointingly low. Such catalysts are of no commercial interest for solution processes. Other catalysts may exhibit acceptable overall activity at the higher temperatures of a solution process but show tendencies to yield polymers of broad molecular weight distribution or of too low a molecular weight to be commercially useful for the manufacture of a wide range of useful products. The requirements for and the performance of a catalyst in a solution polymerization process are quite different from those of a catalyst in a low temperature polymerization process, as will be understood by those skilled in the art.
A process for the preparation of polymers of ethylene at high temperatures in which the catalyst is a coordination catalyst consisting of titanium tetrachloride, a vanadium compound and aluminum trialkyl and capable of being used in a solution process is described in Canadian Patent No. 635 823 of D. B. Ludlum, N. G. Merckling and L. H. Rombach, which issued Feb. 6, 1962. The preparation of polymers of ethylene in the presence of heat-treated polymerization catalysts is described in published European patent application No. 57 050 of V. G. Zboril, M. A. Hamilton and R. W. Rees, published Aug. 4, 1982, and No. 56 684 of V. G. Zboril and M. A. Hamilton, published July 28, 1982. The preparation of polymers of ethylene in the presence of polymerization catalysts containing alkyl siloxalanes is described in European patent application No. 131 420 of M. A. Hamilton, D. A. Harbourne, C. G. Russell, V. G. Zboril and R. Mulhaupt, published Jan. 16, 1985.
Aluminoxanes have been used in the preparation of supported catalyst precursors, which are then activated to obtain the polymerization catalyst, as is disclosed in East German Patent No. 105 244 of Polyakov Z. N. which was published Apr. 12, 1974, U.S.S.R. Patent No. 749 421 of Polyakov Z. N. which was published July 25, 1980, and U.S.S.R. Patent No. 520 127 of A. I. Gorbunov et al. which was published Oct. 29, 1976. In addition, aluminoxanes have been used in zirconium-based homogeneous coordination catalysts for low temperature polymerization processes.